228 LORANDE LOSS WOODRUFF 
Jennings, in his studies on heredity in Paramaecium,!* showed 
that he could readily isolate a considerable number of pure lines 
from a wild culture, and that these pure lines breed true, 1.e., 
there exist inherent hereditary differences in size, persisting when 
all other conditions remain the same. ‘These different lines fall 
usually into two main groups, one group having a mean length 
greater than 170u, and the other having a mean length less than 
140u. But he was able finally to isolate a line intermediate in 
size, and thus to bridge over the gap. As Jennings points out, 
even if it were not possible to isolate a strain of intermediate 
size between the two large groups, this would not give a basis 
for distinguishing two species. However, he states: “I may 
be permitted to add to the precise data thus far given a personal 
impression or surmise. Though, as I-have shown, intermediate 
lines occur, I believe that it will be found that most Paramecia 
can be placed in one of the two groups that we have called ‘cau- 
datum’ and ‘aurelia’. In other words, if my impression is cor- 
rect, most lines will have a mean length either below 145 microns 
or above 170 microns; rarely will lines be found whose mean falls 
between these values. Such at least has been my experience in 
a large amount of work. Furthermore, I am inclined to believe 
that those belonging to the smaller group (mean length below 
145 microns) will be found to have as a rule two micronuclei; 
those belonging to the large group but one micronucleus. This 
matter is worthy of special examination.” 
Jennings and Hargitt in 1909 made this examination and in a 
preliminary communicatior’ stated!® that “two sets of races 
could be distinguished, one set having two micronuclei, the other 
but one. The races with two micronuclei were all smaller than 
those with one. The larger races together thus correspond with 
what had before been described as P. caudatum, the smaller races 
with P. aurelia. The two differ also in the size, position and 
18Heredity, variation and evolution in Protozoa. II. Heredity and variation of 
size and form in Paramecium, with studies of growth, environmental action 
and selection, Proc. Amer. Philosophical Society, 47, no. 190, 1908. 
19Characteristics of the diverse races of Paramecium, Proc. Amer. Soc. Zool- 
ogists, 1909 meeting, in Science, March 25, 1910. 
